Apolipoprotein B (apoB) mRNA is modified by a post-transcriptional editing reaction (C to U) changing a glutamine (CAA) to a translational stop codon (UAA) and producing apoB-48 mRNA in mammalian liver and intestine. Developmental and age-related changes in apoB mRNA editing were studied using two mouse strains with different aging processes (SAM-R/1 with a normal aging process and SAM-P/1 with an accelerated aging process). During growth of both strains, the proportion of unedited (apoB-100) mRNA decreased from 80% in the fetal liver at the 17th day of gestation to 30% in the liver of mature 2-month-old mice. Age-associated increase in the proportion of hepatic apoB-100 mRNA was observed from the age of 18 months in the SAM-R/1 strain. In the SAM-P/1 strain, apoB-100 mRNA in the liver continued to increase from the age of 10 months to death. The profiles of developmental and age-related changes in the proportion of two serum apoB isoproteins (apoB-100 and apoB-48) followed the extent of hepatic apoB mRNA editing. Age-related changes in the extent of apoB mRNA editing in the small intestine were not observed in either strain. A slight expression of apoB was detected by reverse transcriptase-polymerase chain reaction in the kidney, stomach, and colon, and age-associated change in the extent of editing was observed in the kidney. These correlated changes in apoB mRNA editing and serum apoB proteins suggest that RNA editing may be one mechanism involved in the regulation of lipoprotein biogenesis in biological development and in senescent mice. An age-associated decrease in the extent of hepatic apoB mRNA editing and increases of the proportion of serum apoB-100 protein were observed in senescent mice.